The present invention relates generally to optical inspection systems for detecting defects on a sample. More specifically, it relates to mechanisms for filtering noise from the detection of defects within such optical systems.
Many instruments currently available for detecting small particles on wafers, reticles, photo masks, flat panels and other specimens use darkfield imaging. Under darkfield imaging, flat, specular areas scatter very little signal back at the detector, resulting in a dark image, hence the term dark field. Surface features and objects that protrude above the surface scatter more light back to the detector. In darkfield imaging, the image is normally dark except areas where particles or circuit features exist. A darkfield particle detection system can be built based on the assumption that particles scatter more light than circuit features.
In darkfield type optical inspection systems, an intense light beam in the visible wavelength range is directed towards a sample. Light scattered from the sample in response to such incident beam is then collected by a detector. The detector generates an image of the sample from the scattered light. Since defects, such as particles or voids, cause the incident light to scatter, scattered light may indicate the presence of such a defect. However, other features of the sample that are not defects may cause the incident beam to scatter resulting in the detection of xe2x80x9cfalsexe2x80x9d or xe2x80x9cnuisancexe2x80x9d defects. For example, repeating patterns on the sample, which are typically present on a semiconductor device, cause incident light to scatter so that sharp bright spots are imaged on the detector. These sharp bright spots may obscure actual defects. Additionally, line features at specific angles on a sample may result in scattering in broad regions of the image at specific angles, e.g., 45xc2x0 and 90xc2x0, which also obscures detection of xe2x80x9creal defects.xe2x80x9d
There are instruments that address some aspects of the xe2x80x9cnuisance defectxe2x80x9d problems associated with darkfield. One method in use today to enhance the detection of particles is spatial filtering. Under plane wave illumination, the intensity distribution at the back focal plane of a lens is proportional to the Fourier transform of the object. Further, for a repeating pattern, the Fourier transform consists of an array of light dots. By placing a filter in the back focal plane of the lens which blocks out the repeating light dots, the repeating circuit pattern can be filtered out and leave only non-repeating signals from particles and other defects under certain ideal conditions.
Although conventional Liquid Crystal type spatial filters work well within inspection systems that operate in the visible light range, they fail to effectively inhibit light in the ultraviolet (UV) region from nuisance sources from reaching the detectors. A UV light source may be used for any number of reasons, e.g., to effectively detect smaller sized defects. However, a conventional spatial filter""s extinction capability is greatly reduced when going from a visible to an UV light source. Additionally, a conventional spatial filter fails to effectively transmit scattered light in the UV region. In a specific example, a conventional PSF has an extinction value of 200:1 and a transmission value of 20 percent at the UV wavelength of 364 nm. It should be noted that the extinction ratios described herein are measured with a detector that is at a distance of 10 inches from the filter and has an aperture of 1 centimeter. Finally, both the transmission and extinction performance of conventional spatial filters degrade over time under UV light exposure.
Accordingly, there is a need for an improved liquid crystal type programmable spatial filter for use in a darkfield optical inspection system that has improved extinction and transmission performance in the UV region.
Accordingly, mechanisms are provided for selectively filtering spatial portions of light emanating from a sample under inspection within an optical system. In one embodiment, a programmable spatial filter (PSF) is constructed from materials that are compatible with light in a portion of the UV wavelength range. In a specific implementation, the PSF is constructed from a UV compatible material, such as a polymer stabilized liquid crystal material. In a further aspect, the PSF also includes a pair of plates that are formed from a UV grade glass. The PSF may also include a relatively thin first and second ITO layer that results in a sheet resistance between about 100 and about 300 xcexa9 per square.
In a specific embodiment, an optical inspection system for detecting anomalies on a sample is disclosed. The system includes a light source for directing an incident light beam onto a sample and a programmable spatial filter (PSF) arranged in a path of light emanating from the sample in response to the incident light beam. Preferably, the PSF being constructed from materials having one or more properties that are configurable to inhibit at least a first portion of the emanating light with a minimum extinction value of about 400:1 and transmit at least at least a second portion of the emanating light with a minimum transmission value of about 40 percent for an incident light beam having a wavelength between about 340 nanometers and about 400 nanometers. Most preferably, the PSF has a minimum extinction value of 500:1 and a minimum transmission value of 50 percent for the same wavelength range. The optical inspection system further includes a detector arranged within the path of the emanating light so that the second portion of the emanating light that is transmitted by the PSF impinges on the detector to thereby form an image of at least a portion of the sample and an analyzer for receiving the image and determining whether there are any defects present on the sample portion by analyzing the received image
In a specific embodiment, the PSF is formed from a pair of plates formed from a material that substantially transmits ultraviolet light and are sized to cover an aperture of the emanating light. The plates are arranged parallel to each other. A first indium tin oxide (ITO) layer is deposed on a first one of the plates, and a second plurality of ITO layer portions is deposed on a second one of the plates. The first ITO layer and the second ITO layer portions are positioned between the plates. The PSF also has a liquid crystal layer arranged between the first ITO layer and second ITO layer portions. The analyzer is further configured to selectively apply a voltage potential difference between at least a one of the second ITO layer portions and the first ITO layer so that an adjacent portion of the liquid crystal layer allows transmission of a first portion of the emanating light while another portion of the liquid crystal layer on which a voltage potential difference is not applied inhibits a second portion of the emanating light through the PSF.
In a further implementation, the liquid crystal material is a UV compatible material. For example, the liquid crystal is a polymer stabilized liquid crystal material. In a preferred embodiment, the pair of plates are formed from a UV grade glass. In another specific implementation, the first and second ITO layers have a thickness value that provides suitable conductance performance, while providing suitable transmission of UV light. For instance, a thickness that is selected to provide a sheet resistance between about 100 and about 300 xcexa9 per square conducts well and provides acceptable transmission values (e.g. the transmission ranges given above).
In a preferred embodiment, the analyzer is further configured to determine which one or more noise portions of the emanating light result from repeating patterns on the sample, and the voltage potential difference is selectively applied to inhibit such noise portions. In a specific aspect, the noise portion(s) correspond to one or more sharp, bright diffraction spots resulting from repeating patterns on the sample. In an additional aspect, the noise portion(s) of the emanating light correspond to one or more regions at one or more selected angle(s) resulting from repeating lines on the sample.
In an alternative embodiment, an optical inspection system for detecting anomalies on a sample is disclosed. The system includes a light source for directing an incident light beam onto a sample and a programmable spatial filter (PSF) arranged in a path of light emanating from the sample in response to the incident light beam. The PSF has a pair of plates formed from a material that substantially transmits ultraviolet light and are sized to cover an aperture of the emanating light. The plates are arranged parallel to each other. A first indium tin oxide (ITO) layer is deposed on a first one of the plates, a second plurality of ITO layer portions is deposed on a second one of the plates. The first ITO layer and the second ITO layer portions are positioned between the plates, and a polymer stabilized liquid crystal (PSLC) layer is arranged between the first ITO layer and second ITO layer portions.
The system further includes a detector arranged within the path of the emanating light so that the second portion(s) of the emanating light that are transmitted by the PSF impinges on the detector to thereby form an image of at least a portion of the sample. The system also includes an analyzer for receiving the image and determining whether there are any defects present on the sample portion by analyzing the received image. The analyzer is further configured for selectively applying a voltage potential difference between at least a one of the second ITO layer portions and the first ITO layer so that an adjacent portion of the PSLC layer allows transmission of a first portion of the emanating light while another portion of the PSLC layer on which a voltage potential difference is not applied inhibits a second portion of the emanating light through the PSF.
In another aspect, the invention pertains to a method of detecting anomalies on a sample. An incident light beam is directed onto a sample. The incident light beam has a wavelength between about 340 and 400 nm. One or more first spatial portion(s) of an emanating light beam, which results from the incident light beam hitting the sample, are selectively inhibited from reaching a detector positioned within an image plane of such emanating light beam. The inhibition is accomplished by configuring a programmable spatial filter (PSF). One or more second spatial portion(s) of the emanating light beam are transmitted through the PSF to the detector. The first spatial portion(s) have a minimum extinction value of about 400:1 and the second spatial portion(s) have a minimum transmission value of about 40 percent.
In one aspect, the first spatial portion(s) that are inhibited from reaching the detector are selected to correspond to emanating light resulting from repeating patterns on the sample. The first spatial portion(s) are selected to correspond to one or more sharp bright diffraction spots resulting from repeating patterns on the sample. In another aspect, the first spatial portion(s) are selected to correspond to one or more regions at one or more selected angle(s) (e.g., 45 and/or 90 degrees) resulting from repeating patterns or lines on the sample.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.